fix: mpv rendering issue (black screen) and export more xdg related env vars

Hi Marcus,

Was really busy with work for sometime now. Finally got some time to go through some issues and fix them.

1. Now you can set video background with libmpv rendering. Tested on X11 without issues through sway (as mentioned in README). The call to loadfile was happening before render initialization in mpv which caused various issues.
2. Exporting some XDG env vars as I saw in some other loging manager such as yl and uwsm. Haven't really looked deep into their source yet.

Along with the qtgreet_play.py script I shared in this issue, I now have an offline playlist, and everything is running so smoothly that I am reminded again of how good this software is. Really good work.